Build default.vlt in a reproducible fashion (fixes #4562)

gzip was including a timestamp, and tar was enumerating files in an
unspecified/variable order.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(+ minor modifiations)

share/Makefile.am

@@ -82,7 +82,10 @@ skins2_default_vlt_FILES = \
 
 skins2/default.vlt: $(skins2_default_vlt_FILES)
 	mkdir -p skins2
-	tar cvvzf skins2/default.vlt -C $(srcdir)/skins2 default/
+	(cd "$(srcdir)/skins2"; find default -print0 | \
+		LC_ALL=C sort -z | \
+		tar cvv --exclude .svn --no-recursion --null -T -) | \
+	gzip -n > skins2/default.vlt
 
 DIST_http = \
 	http/.hosts \